Research highlight - Opalinus Clay & Class G cement experiments at EIFER

The loss of well integrity may often be explained by the altered geochemical equilibrium between caprock/cement rock-forming minerals and formation water favouring dissolution/precipitation processes. The driving factor is the dissolved CO2 in the brine which is dependent on temperature, pressure and salinity of the brine. To tackle this issue laboratory experiments were conducted at EIFER.

 

The impact of CO2 leads to carbonate (Cc) dissolution reactions for the Opalinus Clay. This can be observed with XRD (Fig.1, left) and gasometric analysis of Opalinus clay samples comparing CO2 and N2 autoclave experiments. Also fluid analysis is indicative with regard to carbonate dissolution (Fig.1, right) showing increased values for Ca2+ and HCO3-. Experiments were conducted at conditions similar to the Mont Terri URL experiment (20 bar total pressure & 30°C) and at conditions assuming a shallow reservoir (100 bar total pressure & 40°C), quite similar to Sleipner in-situ conditions. Both times carbonate dissolution occurs with an slightly enhanced dissolution for the 100 bar, 40°C experiments.

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 For the Class G cement the major observation is the carbonation process for both pressure/temperature combinations as can be observed with XRD analysis (Fig.2). Same as for the Opalinus Clay, the reactivity appears to be enhanced for the 100 bar, 40°C experiments which can be explained by the enhanced CO2 solubility. One feature of the 100 bar, 40°C experiments is that the carbonation process leads to formation of calcite (Cc) and aragonite (Ag), which are chemically equivalent but with different crystal structure. For the 20 bar, 30°C experiments only calcite occurs. This can be explained by the temperature difference of the experiments, whereby aragonite is generally favoured above ca. 30°C.

 This work shows that the investigated materials, (1) Opalinus Clay, and (2) a Class G well cement, show relevant geochemical reactions which need to be considered for a complete understanding of the near-well sealing integrity. The bottom line is that the near-well integrity issue with regard to the geochemistry is an interaction of single allegedly positive and negative effects which need to be considered in the overall near-well context i.e. also including hydraulics and mechanics, to understand and reliably evaluate the sealing efficiency of wells in the context of CO2 storage.  

The nature of occurring reactions, for both, low pressure/temperature and high pressure/temperature experiments, shows no discrepancies, therefore Mont Terri URL experiment conditions can be qualitatively compared to regular CO2 storage conditions. With regard to the quantity of occurring reactions care has to be taken, when extrapolating the results of the Mont Terri URL experiment to regular CO2 storage conditions as variations can occur. Considering this difference of quantity is relevant for a reliable evaluation of the near-well sealing integrity in CO2 storage context.

 

Dernière mise à jour le 29.07.2015